MALAC0ZOA. TROPIOFODA. LAMELLIBRANCHIATA. 285 



which are often obliterated in old individuals. Liga- 

 ment external. Muscular impressions distant, round- 

 ish, the anterior smaller. 



Individuals of the same species vary exceedingly in 

 form. 



The Saxicavse live immersed in sand, mud, clay, frag- 

 mentary masses, and cavities in rocks. 



1. Saxicava rugosa. Wrinkled Saxicava. 



Shell oblong, elliptical, or obovate, regular or variously dis- 

 torted, thick, coarsely and irregularly rugose, covered with a 

 greyish-brown or yellowish epidermis ; the umbones rather 

 prominent; a triangular space behind bounded by a promi- 

 nent line from the umbo to the posterior angle ; the anterior 

 end short and rounded, the postei'ior prolonged, and angulate 

 or rounded ; ligament large and prominent. 



Young individuals have the posterior end angulate, the ridge 

 from the umbo to the angle, and the dorsal margin lamelloso- 

 spinous. The older the shell, the more prolonged and 

 rounded the posterior end. Often also variously distorted. 



From the variety of appearances which it thus assumes, it 

 has been described under various names. Its variableness, as 

 to form and thickness however, have, I think, caused it by 

 later authors to be confounded with a species which appears 

 to me to be perfectly distinct. 



Mytilus rugosus. Linn. Svst. Nat. i. 1156. Penn. Brit. Zool. 

 iv. 110. PI. 63. f. 72.— Saxicava rugosa. Turt. Brit. Biv. 20. 

 PI. 2. f. 10. — Saxicava rugosa. Lamk. Syst. v. 501; Ed. 2. 

 vi. 152. — Saxicava gallicana. Lamk. Syst. v. 501 ; Ed. 2. vi. 

 152. — Saxicava pholadis. Lamk. Syst. v. 502; Ed. 2. vi. 152. — 

 Saxicava pholadis. Tint. Brit. Biv. 21. PI. 2. f. 11.— Hiatella 

 rugosa. Fleni. Brit. Anim. 461. — Alvtilus rugosus. Mont. Test. 

 Brit. 164. 



2. Saxicava drclica. Arctic Saxicava. 



Shell oblong, or linear-oblong, regular or distorted, thin, 

 coarsely and irregularly rugose, covered with a yellowish-grey 

 epidermis ; the umbones rather prominent ; the frontal end 

 very short, angulate ; the dorsal line elongated, straight, the 

 posterior end abrupt or rounded ; an elongated triangular 

 space behind and above, with two prominent spiniferous 

 ridges, from the umbo to the two angles ; ligament short and 

 little prominent. 



Although individuals of this species may resemble those of 



